The present invention relates to an electrical isolation system for a fuel cell stack and to a method of operating a fuel cell stack.
Fuel cell stacks comprise a plurality of fuel cells connected in series and/or in parallel. There are many different designs of fuel cells, some of which operate at extremely high temperatures and others of which operate at relatively low temperatures. Fuel cells, which operate at relatively low temperatures, tend to be preferred for use as power plants in vehicles. There are various types of low temperature fuel cells. One frequently used type of fuel cell for vehicle applications is the so-called PEM fuel cell (Proton Exchange Membrane). In a fuel cell of this kind, an anode electrode and a cathode electrode both coated with catalyst material are separated by a synthetic membrane and the assembly comprising the two electrodes separated by the membrane, frequently called an MEA (membrane electrode assembly) is enclosed between two conductive plates referred to as bipolar plates. In a fuel cell stack a plurality of fuel cells are arranged side by side so that each bipolar plate (apart from the end plates of the stack) is associated with two adjacent fuel cells. The bipolar plates are provided at their sides facing the electrodes with passages or channels which enable hydrogen to be fed to the anode electrode of one fuel cell and oxygen in the form of air to be fed to the cathode electrode of a neighboring fuel cell. When the fuel cell is in operation the protons delivered by the hydrogen migrate through the membrane and combine with the oxygen to form water and generate electricity. When a plurality of fuel cells are arranged in a stack, the bipolar plates serve as a separator between adjacent fuel cells, that is to say the bipolar plate has at one side passages for directing hydrogen to the anode of one fuel cell and at the other side passages for directing air to the cathode of an adjacent fuel cell and keeps these gas flows separated.
In the operation of such fuel cells, heat is generated and provision is made for cooling the fuel cells. This cooling is effected by incorporating cooling passages into the bipolar plates through which a coolant flows. Thus, the bipolar plates have a separating function in that they separate adjacent fuel cells. At the same time, they are connected together electrically, in series and/or in parallel, in order to connect them into a power circuit by which the electricity generated by the fuel cell can be extracted. A typical PEM cell produces an output voltage of about 0.9 V. In a typical fuel cell stack there are a sufficient number of fuel cells to produce a relatively high operating voltage, typically in the range from 100 to 400 V. Fuel cells with high operating voltages are the subject of stringent safety requirements, particularly when liquid coolants are used to cool the fuel cell stack. Previous attempts to meet these requirements have focused on trying to achieve complete isolation of the coolant circuit involving radiators, pumps, tubes as well as complete isolation of the fuel cell stack itself. Attempts have also been made to use non-conductive liquids as the coolant, which is intended to prevent dangerous voltage levels at the fuel cell stack being transmitted by the coolant to the radiator and other components which would prevent a serious safety hazard.
The electrical isolation of large components, such as radiators, is however not very practical in a vehicle or in any other system due to size constraints and problems associated with the blocking of cooling air. The use of non-conductive coolants (for example oil) has significant disadvantages because the physical properties of such coolants, such as heat capacity, heat conductivity, and viscosity, are restricted. Moreover, such non-conductive coolants pose an environmental problem since there is always the danger of leakage, for example if connections fail or in the event of accident damage Moreover, there are particular problems in operating such coolants at low temperatures. Such disadvantageous properties adversely affect the system power density the radiator size and the power required to drive radiator fans and coolant pumps.
Because of these disadvantages attention has been paid to using water plus anti-freeze based coolants for liquid cooling. However, it is important to use a coolant with a relatively low conductivity. As explained above the bipolar plates of the fuel cells of the fuel cell stack are connected electrically in series and/or parallel and the liquid coolant flows in parallel through the bipolar plates. Thus, if the liquid coolant is conductive it effectively represents a ground fault of the bipolar plates, which is clearly undesirable.
Liquid coolants are available with a relatively low conductivity favorable for use in fuel cells. However, there is always the danger, in the practical use of a fuel cell system, that someone could add the wrong coolant to the system. The liquid coolants used in fuel cell stacks are also critical from the point of view that they must be designed to avoid corrosive and electrolytic effects, which could lead to long-term deterioration of the fuel cell stack.
Moreover, it is known that liquid coolants deteriorate in use over a longer period of time.
In addition to the aforementioned problems there is also a general problem with fuel cell stacks in as much as faults can occur which lead to a deterioration or failure of the isolation of the fuel cell stack, which could lead to dangerous situations. Such dangerous situations could be particularly acute if the vehicle has been involved in an accident or if some other malfunction has taken place which impairs the quality of the isolation.
In view of the above mentioned problems, it is an object of the present invention to provide an electrical isolation system for a fuel cell stack and a method of operating a fuel cell stack such that the quality of the electrical isolation can be continuously monitored and such that safety measures can be taken in the event of faulty isolation to prevent damage to the fuel cell stack and associated components and to prevent dangerous situations due to inadequate electrical isolation.
Moreover, it is a further object of the present invention to make available an electrical isolation system and a method of the above named kind which can be implemented at relatively low cost and which operates reliably.
It is a yet further object of the present invention to provide an electrical isolation system and a method of the above named kind which enables a realistic approach to be taken to considerations such as deterioration of the liquid coolant which necessarily occurs over a period of time, with it being possible to ensure the liquid coolant is changed on time before deterioration has reached a critical level.
In order to satisfy these objects there is provided, in accordance with the present invention, an electrical isolation system for a fuel cell stack comprising a plurality of fuel cells connected in series and a coolant circuit for cooling said fuel cells in operation using a liquid coolant having a restricted electrical conductivity, said fuel cell stack being associated with a chassis having a chassis ground and comprising a plurality of coolant passages for said fuel cells, said coolant passages being connected in parallel and/or in series and said coolant circuit comprising an inlet for feeding said liquid coolant into said stack and into said coolant passages, an outlet for removing said liquid coolant from said stack after flow through said coolant passages, a radiator provided as a heat exchanger to cool said liquid coolant and having an inlet and an outlet, a first coolant flow line connecting said radiator outlet to said fuel cell stack inlet, a second coolant flow line connecting said stack outlet to said radiator inlet and a pump for circulating liquid coolant in said coolant circuit, wherein said coolant circuit comprises a plurality of conductive components such as an outer boundary wall of said fuel cell stack, said radiator and/or said pump, wherein at least one of said conductive components is connected to said chassis ground and wherein a measuring circuit is provided for measuring the resistance between a selected one of said fuel cells and said chassis ground.
The outer boundary wall of the fuel cell stack may, for example, be a wall of a metallic housing surrounding the stack, or a metal panel or structure of a vehicle adjacent to the stack or an end plate or side wall of the stack itself.
Also there is provided a method of monitoring a fuel cell stack comprising a plurality of fuel cells connected in series and a coolant circuit for cooling such fuel cells in operation using a liquid coolant having a restricted electrical conductivity, said fuel cell stack having an associated electrical output system, at least one output terminal and a contactor for connecting each said output terminal to said electrical output system and being associated with a chassis having a chassis ground, said fuel cell stack further comprising a plurality of coolant passages for said fuel cells, said coolant passages being connected in parallel and/or in series and said coolant circuit comprising an inlet for feeding said liquid coolant into said stack and into said coolant passages, an outlet for removing said liquid coolant from said stack after flow through said coolant passages, a radiator provided as a heat exchanger to cool said liquid coolant and having an inlet and an outlet, a first coolant flow line connecting said radiator outlet to said fuel cell stack inlet, a second coolant flow line connecting said stack outlet to said radiator inlet and a pump for circulating liquid coolant in said coolant circuit, wherein said coolant circuit comprises a plurality of conductive components such as an outer boundary wall of said fuel cell stack, said radiator and/or said pump, where at least one of said conductive components is connected to said chassis ground, the method comprising the steps of measuring a resistance between a selected one of said fuel cells and said chassis ground and effecting a comparison, directly or indirectly, between said measured resistance and at least one threshold value and, in the event of an unfavorable comparison, generating a warning signal and/or disengaging any contactor connecting a said output terminal of said stack to said electrical system and/or shutting down said fuel cell stack.
Whenever reference is made in this specification and claims to items in the singular, such as xe2x80x9ca radiatorxe2x80x9d, xe2x80x9ca pumpxe2x80x9d, xe2x80x9ca contactorxe2x80x9d etc. it will be understood to mean one or more such items.
The concept underlying the present invention is thus first of all the selection of a lay-out of the fuel cell stack and of the associated electrical system which makes it possible to relate changes in resistance to changes in the quality of the electrical isolation of the fuel cell stack and of the associated system and which also makes it possible to analyze the reasons for the change in electrical resistance and thus to take appropriate remedial action (warning and/or disconnection and/or shut down).
For example, a gradual change in the measured resistance can be associated with a gradual deterioration of the coolant and can lead to a warning signal being given when the coolant needs changing. If the deterioration is about to reach a critical level then the fuel cell stack can be automatically shut down.
If a sudden change in resistance occurs then this can be due to a number of reasons. For example the vehicle may have had an accident or even just a slight bump which has led to a ground fault within the fuel cell stack or within the electrical system which can lead to a characteristic change of the measured resistance.
Equally, if an object inadvertently comes into contact with a part of the fuel cell stack or associated coolant system or the associated electrical system which should be insulated from contact, but is inadequately insulated for whatever reason, for example because a cover has been omitted or because of accident damage, then a resistance will effectively be placed in parallel with the measured resistance of the cell and a characteristic change will take place.
On detecting such a sudden change one or more electrical contactors can immediately be actuated to break the electrical circuit, i.e. to disconnect the high voltage terminal or terminals of the fuel cell stack from the electrical system, thus preventing dangerous situations. Moreover, the fuel cell stack can be shut down, i.e. valves can be actuated to cut of the supply of hydrogen and or atmospheric oxygen to the fuel cell stack to inhibit the generation of electricity and/or the stack can be purged of combustible gases.
In other words, if a tool has been left within the environment of the fuel cell and has caused a short circuit or a ground fault this will immediately result in a change in the measured resistance which will be detected and the appropriate remedial action can be taken, such as activating the contactors to break the electrical circuit and or shut down the stack. That is to say the measured resistance value can be used in a manner analogous to a differential protection system (FI switch).
Moreover, if the cooling system is damaged for some reason, for example such that the flow of coolant is restricted, then this will have an effect on the measured resistance. This can be detected and again a warning signal can be issued or the system can be shut down depending on the severity of the change. Similarly, if a ground connection is missing, or corroded, or has broken or been forgotten, then this will have an effect on the measured resistance and can thus be detected.
Should someone inadvertently add a liquid coolant with incorrect electrical conductivity to the system during servicing or on topping up, then this will also result in a change in the measured resistance value and an appropriate warning signal can be issued, or the fuel cell system can be shut down, if the change in conductivity of the coolant is critical.
It is particularly preferred when the electrical isolation system also includes a circuit for measuring a potential difference between the selected one of said fuel cells and the chassis ground.
The selected one of the fuel cells is preferably the first fuel cell adjacent the stack inlet and or outlet for coolant (but need not be the first fuel cell). The bipolar plate of the first fuel cell closest to the stack inlet and the stack outlet may for example settle at an operating potential difference in the range from +20V to xe2x88x9220 V relative to ground, with this potential difference depending on the conductivity of the coolant, on ground fault currents and on cooling geometry effects. By monitoring this potential difference in addition to the measured resistance it is possible to obtain further information concerning the quality of the electrical isolation of the fuel cell stack and associated electrical system and to improve the analysis of the reasons for changes in the electrical isolation. This enables better evaluation of changes that occur and better decision making in response to such changes.
Thus, the present invention also proposes the use of a combination of a resistance monitoring device which continuously measures and monitors the resistance of the resistive path formed by the coolant and other parasitic resistive isolation paths (referred to as R-ISO between the selected fuel cell and ground) together with a voltage monitoring unit which monitors the voltage across the resistive path from the fuel cell through the passivation layer of the respective cell, across channel areas in the MEA and across coolant flow paths in the fuel cell stack and in the coolant manifold towards the chassis ground (referred to as V-ISO). Preferably the fuel cell in closest contact to the coolant inlet/outlet is selected for the connection to the monitoring circuit.
This makes it possible to realize a control device and to implement an algorithm which is capable of calculating not only a change in resistance but also fault currents flowing along the described coolant path, in particular DC fault currents and low frequency AC fault currents. This fault current can be calculated using Ohms law, i.e.,
I-ISO=V-ISO/R-ISO
and makes it possible to compare each of R-ISO, V-ISO and I-ISO against adaptive safety thresholds and to initiate warnings and/or high voltage shut down. This system makes it possible to take account of and supervise coolant deterioration over time, undesired variations in coolant channel geometry, and loss of safety grounding.
The preferred use of a stack coolant scheme with a stack or stack arrangement where a low conductivity coolant enters and exits the stack at the same voltage potential plate, which may, for example, be an end plate of the fuel cell stack or a center tap plate for multiple stack arrangements, makes it possible to form a controlled grounding path. All conductive elements of the coolant circuit that are in contact with the coolant, where the danger exists they might go to hazardous voltage levels in the case of an isolation fault, are connected to the chassis ground and thus grounded safety-wise.
The electrical isolation system is preferably laid out in such a way that the resistive path between the selected one of the fuel cells and the chassis ground leads to a measured resistance which is as high as possible. With an arrangement of this kind changes in resistance can be measured sensitively.
Preferred embodiments of the electrical isolation system and method of the invention are set forth in the subordinate claims and will now be described in further detail with reference to the accompanying drawings in which are shown: